There is a problem of the increase in heat generation caused by development of a high-power downsized semiconductor element with an electric power control. A conventional semiconductor device forms a module in which a semiconductor element is disposed on a substrate made of an electrical insulator so as to electrically isolate the semiconductor element, and in which the electrically insulating substrate is disposed on a heat sink (a heat dissipating plate) and a cooling plate.
AlN (described, for example in Japanese Patent Application Laid-Open (JP-A) No.7-99268) or Al2O3 is widely used as a heat sink material which should provide electrical insulation and thermal conductivity.
An example of the structure of the semiconductor device using a ceramic substrate such as AlN disclosed, for example, in JP-A No.7-99268 is a structure in which: conductive layers such as Cu plates are brazed onto both sides of an electrically insulating substrate made of AlN, the Cu plate on one of the sides is plated with Ni; a semiconductor element is provided on the Ni-plated surface; and a dissipating plate is soldered onto the other Cu plate.
In another example, conductive layers made of Al are provided on both sides of an electrically insulating substrate made of AlN; a semiconductor element is provided on the Al conductive layer on one of the sides; a heat dissipating plate made of Cu—Mo or Al—SiC capable of reducing thermal stress is soldered onto the other Al conductive layer; and an Al cooling plate is attached onto the heat dissipating plate by grease.
However, since a conventional semiconductor device using ceramics such as AlN described above has a complicated and multi-layered structure, heat dissipating property is not satisfactory, and the producing cost is high.
It has been proposed (see JP-A Nos. 11-292692 and 2000-174166) to use a gas-phase synthesized diamond (insulating hard crystalline carbon film) substrate or a substrate obtained by coating AlN with a gas-phase synthesized diamond as a heat sink so as to improve the heat dissipating property of the heat sink.
When a diamond with a high thermal conductivity is used as the heat sink, the heat-dissipation property is improved; however, the following problems occur: (1) since the surface of the diamond grown by the gas-phase synthesis method has significant unevenness, the surface needs polishing and it is very difficult to secure a sufficient surface smoothness; (2) peel-off occurs easily, a surface activation treatment has to be conducted in advance in order to allow diamond to grow thereon (the surface activation treatment may be roughening of the surface of the electrically conductive substrate by diamond polishing), and it is impossible to grow diamond on the surfaces of some conductive substrates (for example, it is impossible to form diamond on Al or Cu); (3) productivity is low and producing cost is high because of a slow film-forming rate; and (4) diamond has poor compatibility with other material whereby it is not easy to make an electrode comprising diamond.
On the other hand, another example has been reported (see, JP-A No. 10-32213). In the example, the protective layer of the semiconductor element is an electrically insulating amorphous carbon film, which has inferior thermal conductivity than diamond but which is easier to produce than diamond. Further, JP-A No. 10-223625 discloses an example in which an electrically insulating amorphous carbon film is used as an electrically insulating film of a semiconductor device. Such a hard amorphous carbon film is a diamond-like carbon (DLC) film, which has been spotlighted as a low-friction sliding material superior in anti-adhesion property and friction resistance.
When the semiconductor element is a power device such as an IGBT element and the semiconductor element has an electrically insulating amorphous carbon film as a protective layer, the electrically insulating amorphous carbon film contributes heat dissipation in a horizontal direction of the semiconductor element; however, the contribution is low compared with the heat dissipation in the vertical direction of the semiconductor element, which is the heat dissipation through the heat sink disposed on the surface on the side opposite to the side having the protective layer for the semiconductor element, so that the heat dissipation property as a whole is not sufficient in some cases.
In addition to the problems described above, it is also important for a semiconductor device to maintain a stable heat dissipation property and a stable electrically insulating property even when the semiconductor device is exposed to extreme conditions. However, it has been reported that peeling-off of a film occurs in a heat spreader comprising multiple films including a diamond film and the like when a semiconductor device is exposed to a cold-hot cycle between −20° C. to 150° C. (see Solid-State Electronics, Vol. 42, No.12, p2199-2208 (1998)). When the peeling-off occurs, heat dissipation property and electrically insulating property deteriorate.
A DLC film can be made at relatively low cost and has a smooth surface because of the amorphous structure thereof. However, a DLC film generally has a hardness of Hv 4000 or higher and a Young's modulus of 600 GPa or higher because the DLC film is used as a sliding material and thus should be resistant to a heavy sliding load. Therefore, the DLC film has high internal stress, and is easy to be fractured due to peeling-off or impact load. In addition, since its defect area ratio is 10−3 or higher, voltage resistance is not sufficient; specifically, it has been difficult to obtain voltage resistance of 5V or higher. Although the voltage resistance can be improved by increasing the thickness of the film, minute fracturing occurs in the film or the film peels off owing to the high internal stress and the weak adhesion to the conductive substrate. Specifically, the maximum thickness is limited to about 5 μm.
As described above, in the conventional members (such as heat sinks) which dissipate the heat generated in semiconductor elements, and in the semiconductor device using the conventional members, it has been difficult to satisfy, at a high level, all of the requirements comprising the dissipation of the heat generated in the semiconductor elements, the adhesion to the semiconductor element, the easiness of forming a thick film, the reduction of producing cost of the heat dissipating member, and the reduction of producing cost of the semiconductor device using the heat dissipating member.